= Arterial blood gas analysis
\b[Arterial blood gas analysis] applies <blood gas analysis> to a sample taken from an <artery>, providing a time-specific observation of systemic arterial gas tensions and acid-base state. <Arterial oxygen partial pressure> concerns oxygen tension, while arterial carbon-dioxide <partial pressure> and <pH> provide different information. The arterial qualifier is a sampling condition, not merely a label attached to any blood result.<Arterial Blood Gas (ABG) Test by MedlinePlus>[What is an arterial blood gas test? measured quantities]{ref}
The measurement chain includes obtaining an identified sample, preserving its gas composition and analyzing it with a checked instrument. Air contamination, fluid contamination and analysis delay can change the specimen after it leaves the circulation. A reproducible analyzer reading cannot repair a specimen that no longer represents the intended physiological state.<AARC Clinical Practice Guideline: Blood Gas Analysis and Hemoximetry: 2013 by Michael D. Davis and others>[Sections 4.6, specimen documentation; 6.1.2–6.1.3, contamination and delay]{ref}
Equal oxygen tension need not mean equal <arterial oxygen content>. An original illustrative carrier model writes
$$
C_{\mathrm a O_2}=h\,H\,s+\alpha P_{\mathrm a O_2},
$$
where $H$ is <hemoglobin> concentration, $s$ its oxygen-bound fraction and $h$ the carrier capacity per gram. Choose $h=1.34$ mL/g, $\alpha=0.003$ mL/(dL·mmHg), $s=0.98$ and $P_{\mathrm a O_2}=100$ mmHg. Samples with $H=15$ and $H=7.5$ g/dL then contain about 20.00 and 10.15 mL O$_2$/dL, respectively. They have identical stipulated tension and saturation but very different oxygen content. The constants define the illustration; these are not patient results or diagnostic thresholds.
Interpretation also depends on the conditions when the sample was obtained. <Inspired oxygen fraction>, ventilator settings when applicable and sampling time help distinguish a changed respiratory state from a changed measurement condition. A later <pulse oximetry> reading is neither a simultaneous sample nor a measurement of carbon-dioxide tension.
<Arterial blood gas analysis> supplies evidence about gas transport and acid-base balance, but no individual number alone identifies the entire cause of an abnormal result. Measured <co-oximetry> and calculated oxygen saturation should also be distinguished when unusual <hemoglobin> species matter.<AARC Clinical Practice Guideline: Blood Gas Analysis and Hemoximetry: 2013 by Michael D. Davis and others>[Sections 1.0 and 6.1.6, measured hemoximetry and calculated saturation limitations]{ref}
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